Answer

Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.

In the Name of Allah, Most Gracious, Most Merciful.

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.

Dear brother, your question is really appreciated as it shows your keen desire to keep yourself away from places that may badly affect your faith and your keenness to earn your livings from a lawful (halal) way.

As for your question, you are highly required to avoid being in such places; you can try to choose the most suitable shift for that or choose a place where you are not exposed to such deeds such as watching naked or mixed exercising people.

In his reply to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:

Exercising in such places where there are other persons, males or females, who have parts of their bodies (which should be covered according to the Shari`ah) uncovered is really problematic.

Under extreme circumstances where they are few and one can lower his gaze, I cannot claim that it is haram. This is only for persons who live in Western countries environments where it is difficult to find male only places of exercise. Showering naked is not permissible.

Of course, one can avoid this and most of such clubs open for extended hours that one can go at odd times when there are almost no females.

Allah Almighty knows best.